The present invention relates to a magnetic disc apparatus in which position control of data data transducers, i.e. electromagnetic data readout heads, is executed by a head position servo system utilizing servo sectors provided upon disc recording surfaces. In particular, the present invention relates to a magnetic disc apparatus having a plurality of recording surfaces formed on one or more recording discs which are rotated in common.
With a prior art magnetic disc apparatus employing such a head position control technique, either a single servo sector or a plurality of servo sectors are provided on each recording surface, i.e. one servo sector or a plurality of servo sectors is provided for each recording track on a recording surface. Head position control, to maintain a transducer precisely above the center line of a track from which recorded data is to be read out, is performed using position data obtained from the servo sectors.
If a recording disc is rotated about an axis which exactly coincides with the central axis of the disc, then a sufficient degree of position control will be achieved if the head position servo system functions to maintain a single portion of a track (i.e. that occupied by a servo sector) precisely aligned below a transducer. In the case of a prior art apparatus employing only one servo sector on a single recording surface, this is the only form of control which is possible. With such servo control operation, the transducer will be aligned above center line of the track portion containing the servo data. However if rotation is not aligned with the disc central axis, i.e. there is some degree of rotational eccentricity due to some factor such as eccentric alignment of the motor drive shaft which rotates the disc, then a certain amount of wobble of the disc will occur during rotation. Thus, with a single servo sector control system as described above, even if a transducer is maintained precisely above the track center line at one point in each disc rotation, the transducer may become substantially offset from the center line at other rotational positions, if there is some degree of disc wobble as discussed above. This problem sets a limit to the minimum spacing which can be established between adjacent tracks, and hence limits the data recording density. It also becomes increasingly severe as the disc rotation speed is increased in order to achieve higher data readout rates.
By using a plurality of servo sectors on each recording surface, it is possible to substantially overcome this problem, i.e. by deriving servo data successively from the servo sectors during rotation and thereby executing head position control such as to maintain a transducer in optimum alignment with a track center line, i.e. optimum with respect to position deviations due to eccentric rotation as well as with respect to an average position deviation of the track (e.g. resulting from effects of thermal expansion). High rotation speeds are made possible, and closer track spacing can be utilized for increased data recording density. However with such a prior art magnetic disc apparatus in which a plurality of servo sectors are provided for each track, an appreciable amount of track recording capacity is occupied by the servo sectors, and this will substantially reduce the available recording capacity. In addition if the recording format is changed, it may be necessary to change the positions and the number of servo sectors.
Furthermore in the case of a magnetic disc apparatus in which a single servo sector is provided for each track, it is necessary for the disc to rotate by as much as one revolution before head position correction can begin to be executed after a head position changeover command has been issued. Thus, such a method presents problems with regard to the amount of time required for readout of recorded data.
It is an objective of the present invention to overcome the problems of the prior art described above, by providing a magnetic disc apparatus whereby position control is executed accurately while enabling a high level of data recording density and moreover whereby it is not necessary to alter the servo sectors if the recording format is changed.